Golf balls having various structures are commercially selling. There are generally solid golf balls represented by two-piece golf ball and thread wound golf balls for round games. The two-piece solid golf balls have excellent flight performance and durability as compared with the thread wound golf balls. Therefore the two-piece solid golf balls occupy the greater part of the golf ball market. However, the two-piece solid golf ball is inferior to the thread wound golf ball in shot feel and controllability at approach shot.
There has been many suggestions in order to further improve performances of the two-piece solid golf ball. For example, in Japanese Patent Kokai publication Nos. 220889/1988, 98949/1994, 194732/1995 and the like, a two-piece solid golf ball, of which performances, such as flight distance (rebound characteristics), durability, shot feel and controllability are improved by mainly adjusting hardness or hardness distribution of the core to a proper range, is suggested. In Japanese Patent Kokai publication No. 127823 /1998, a two-piece solid golf ball, of which performances are improved by adjusting thickness and hardness of the cover, and hardness distribution of the golf ball to proper ranges, is suggested. In Japanese Patent Kokai publication No. 215778/1997, a two-piece solid golf ball, of which performances are improved by adjusting specific gravity of the core or cover, properties of dimple and moment of inertia to proper ranges, is suggested. In Japanese Patent Kokai publication No. 259080/2001, a two-piece solid golf ball, of which performances are improved by adjusting hardness distribution of the core, thickness and hardness of the cover, hardness of the golf ball and properties of dimple to proper ranges, is suggested.
In Japanese Patent Kokai publication No. 220889/1988, a two-piece solid golf ball having a hardness distribution in JIS-C hardness such that a hardness at the distance of less than 10 mm from the center point of the core is within the range of 60 to 79 and a hardness at the distance of more than 10 mm from the center point is within the range of 80 to 90; a thickness of the cover of 1.5 to 2.0 mm; and a deformation amount of the golf ball of 1.9 to 2.4 mm when applying from an initial load of 10 kgf to a final load of 130 kgf; is disclosed. However, in the golf ball, it is problem that the deformation amount of the golf ball is small, that is, the golf ball is hard, and its shot feel is poor.
In Japanese Patent Kokai publication No. 98949/1994, a two-piece solid golf ball having a hardness distribution of the core such that a hardness of the core is higher in order at the center point of the core, at the distance of 5 to 10 mm from the center point, at the distance of 15 mm from the center point and at the surface of the core is disclosed. However, in the golf ball, the cover is formed from relatively hard material, and its flight distance is long, but it is problem that its spin amount when hit by a short iron club and the like is small, which degrades its controllability.
In Japanese Patent Kokai publication No. 194732/1995, a two-piece solid golf ball having a center hardness in JIS-C hardness of the core of 40 to 57, a surface hardness in JIS-C hardness of the core of 70 to 95, and a difference between the surface hardness and center hardness of 20 to 40 is disclosed. However, also in the golf ball, the cover is formed from relatively hard material, and it is problem that its spin amount when hit by a short iron club and the like is small, which degrades its controllability.
In Japanese Patent Kokai publication No. 127823/1998, a two-piece solid golf ball having a thickness of the cover of 1.0 to 2.1 mm, which is thin; a JIS-C hardness of the cover of 75 to 95; and a hardness distribution such that a difference between the cover hardness and a center hardness in JIS-C hardness of the core is not more than 10, which is small; is disclosed. However, in the golf ball, the cover is formed from only ionomer resin, and it is problem that its rebound characteristics is poor, and its flight distance when hit by a driver is short.
In Japanese Patent Kokai publication No. 215778/1997, a two-piece solid golf ball, of which the core has a specific gravity of not less than 1.00; a specific gravity of the cover is higher than that of the core; properties of dimple such as a ratio of ball surface area occupied by the dimple, a dimple space volume and the like, and a relationship between moment of inertia and cover hardness are adjusted to proper ranges; is disclosed. However, in the golf ball, since the specific gravity of the cover is too large, the moment of inertia of the resulting golf ball is very large. Therefore, in case of golf ball having relatively hard cover, since initial spin amount is small, its flight distance is improved by restraining the decrement of the spin amount is restrained. On the other hand, in case of golf ball having relatively soft cover in view of controllability, since retention of spin amount is too high than need be, and the hit golf ball creates blow-up trajectory, which reduces the flight distance.
In Japanese Patent Kokai publication No. 259080/2001, a two-piece solid golf ball, of which the core has a difference between a surface hardness and center hardness of not less than 20; the cover has a thickness of 1.3 to 2 mm and a Shore D hardness of not more than 55; a hardness difference in JIS-C hardness between the surface of the cover and the surface of the core is not more than 0; properties of dimple such as total number of dimples, percent dimple volume and the like are adjusted to proper ranges; is disclosed. However, since soft ionomer resin is used as a cover material, rebound characteristics the resulting golf ball are degraded, which reduces it flight distance. In addition, its scuff resistance is poor.
It is required to provide a two-piece golf ball having excellent flight performance, good shot feel and controllability by solving the problems. Recently, multi-piece solid golf balls having excellent flight performance, good shot feel and controllability, as compared with a conventional two-piece solid golf ball, are developed. Therefore it is required to further improve the flight performance, shot feel and controllability also in the two-piece golf ball.